Field of the Invention
Described in the literature are examples of aqueous polyurethane dispersions which form film at room temperature, of cationic, anionic or non-ionogenic kind. The characteristic picture of these dispersions is very diverse with interesting physical properties and application-technical possibilities.
The chemical resistance and weather stability of dispersion films drying exclusively physically at room temperature, as well as also at higher temperature, can now still be improved in comparison with polyurethane reaction lacquers containing solvents.
In the case of cationic polyurethane dispersions whose technical preparation requires a number of process steps, acid catalyzed after-linkages can be conducted with hydroxyl-containing hardeners. However, the precursors of the resulting films cannot be considered totally satisfactory.
Anionic polyurethane dispersions, which are more easily accessible commercially, cause some difficulties with regard to a later after-linkage since in the alkaline field smoothly proceeding reactions without discolorations in tolerable temperature ranges are not yet known.